In recent years, modern electronic technology has given rise to a wide variety of electric clocks having digital displays. The predominant method of creating digital clock displays has been by the use of light-emitting diodes (LED's). In most instances, these LED digital clocks are in the form of red or green seven-segment bar arrays representing the FIG. 8. By selectively energizing certain groups of bars, one can cause the array to display any of the ten Arabic numerals. The timing information which controls the selective energizing of the groups of bars is maintained by and distributed to the display by solid-state electronic logical devices containing no moving parts.
An alternative to the LED digital clock display is found in the use of optical fibers. Bundles of optical fiber light guides are selectively illuminated at one end and display horological information at the other end. The selective illumination of the proper bundle of optical fibers has been accomplished by using a rotating light source. In one form, the light source itself may be mounted on a shaft which is rotated by a conventional clock motor. Alternatively, a stationary central light source may be directed into one end of a U-shaped light guide while the opposite end is rotated by a conventional clock motor, or a prism and mirror system may be used where the mirror is rotated by a conventional clock motor. Clocks having a digital display produced in the manner just described are shown in U.S. Pat. No. 3,651,634.
There are difficulties associated with clocks having optical fiber displays generated in the manner described above. In a clock where a lamp is mounted on the end of a shaft which is being rotated by a conventional clock motor, electrical power to the lamp must normally be supplied through slip-ring electrical contacts. The problems normally associated with the use of slip-ring contacts, such as contamination, corrosion and wear are therefore a disadvantage of such clocks.
The slip-ring contact problem is greatly compounded if the clock is to include a simultaneous display of hours, minutes and seconds. Three lamps mounted on shafts moving at different speeds must be used, requiring three separate and electrically insulated slip rings to be incorporated onto a single clock motor.
Another limitation in using a rotating light source is that at any one time there is only one number on the time scale which is illuminated. While the U-shaped light guide system and the prism and mirror system disclosed in U.S. Pat. No. 3,651,634 eliminate the slip ring problems, they also illuminate only one number at one time.
There are decided advantages to having a digital display wherein all numbers are simultaneously illuminated except for the numbers indicating the present time, which are either darkened or illuminated in a different color. A digital display of this latter type presents a visual appearance analogous to that of a conventional analog clock face in which the position of the hands relative to the number scale is easily perceived. When the entire clock face is illuminated and visible, the relative position of the colored or darkened number presents an additional indication of time. By contrast, when only a single number is illuminated, the effect is more akin to a digital wrist watch, with the numeral being the only time indicator.